ABSTRACT In normal individuals, food intake is strictly regulated by sensory, homeostatic and hedonic neural circuits, which balance energy intake with energy expenditure. Failure to regulate food perception and appetite result in maladaptive eating behaviors and an increase in the occurrence of metabolic syndromes and eating disorders. Neural circuits that regulate food intake have been extensively investigated in rodent models. However, the complexity of the mammalian brain makes it very challenging to explain the underlying molecular mechanisms and circuit dynamics controlling food intake. I propose to use a genetically tractable model organism, the fly (Drosophila melanogaster), to understand the fundamental principles of how the brain integrates the sensory percept of food with the sensation of hunger to regulate food intake on the level of molecules, cells and circuits. Flies are an excellent model to investigate these processes because they have 1000-fold fewer neurons in the brain than mice, and yet they still show hunger states and specific food intake control remarkably similar to those in vertebrates. Furthermore, the fly nervous system is more accessible for genetic modifications, anatomical studies and monitoring the activity of large populations of neurons in behaving animals. Previously, I have shown that flies, like humans, regulate their food intake by integrating the taste and nutrient value of food with hunger sensation in the nervous system. I identified a novel class of excitatory interneurons (IN1) in the fly brain that regulate food ingestion. In this project, we will first identify the IN1 food intake circuitry using optogenetics and anterograde transsynaptic circuit tracing. Next, we will reveal how IN1 neurons and downstream circuitry change activity during food search in a virtual reality foraging assay using two-photon microscopy. Finally, using cutting- edge three-photon technology, we will capture the activity of IN1 neurons chronically in an intact fly as flies are being food deprived. Functional dissection of IN1 circuitry will lead us to fundamental principles that the nervous system uses to regulate food intake. In parallel with our food intake circuit dissection efforts, we also identified 8 evolutionary conserved genes in a large genetic screen for flies that fail to show compensatory feeding after 24 hours of food deprivation. We will anatomically and functionally dissect the role of these genes and the neural circuits they control in regulating food intake. Finally, we will test the interaction of the candidate food intake genes and the IN1 circuitry in regulating food perception and appetite control. Modelling the food intake and appetite control systematically in a genetically tractable organism allows us to reveal new molecular and neural control mechanisms. Once, we discover key mechanisms underlying food intake and appetite, we can search for similar processes in more complex mammalian models and in patients suffering from obesity or eating disorders to develop treatment strategies that will intervene with the pathogenesis of these life threating diseases.